Wireless communication systems employ various types of modulation schemes. Typically, a modulation scheme may be selected based upon the requirements of the particular system. Orthogonal Frequency Division Multiplex (OFDM) is a modulation scheme that has a primary advantage over single-carrier schemes in that it has an ability to cope with severe channel conditions.
In an OFDM system, the system bandwidth is effectively partitioned into a number (NF) of frequency subchannels that may be referred to as sub-bands, sub-carriers, or frequency bins. Each frequency subchannel is associated with a respective frequency tone. Typically, the transmitted data is encoded with a particular coding scheme to generate encoded bits. The encoded bits may be further grouped into multi-bit symbols that are mapped to modulation symbols based on a particular modulation scheme (e.g., M-PSK or M-QAM). The serial data comprising the mapped modulation symbols are then turned into parallel data symbols with a specific time duration. These parallel data symbols are transformed by an Inverse Fast Fourier Transform (IFFT), which in turn generates the modulation of the data onto the various sub-carriers. The data transmitted on the sub-carriers for each time interval is commonly called an OFDM symbol. Thus, information is transmitted on more than one carrier, which in turn provides frequency diversity and adds robustness.
Nevertheless, each frequency subchannel of an OFDM system may experience different channel conditions (e.g., different fading and multipath effects) and signal-to-noise-and-interference ratios (SNIRs). Thus, the modulation symbols that collectively form a particular data packet may be individually received with different SNIR values. As a result, the supported data rates for the frequency subchannels may also vary over time. Thus, it may be inefficient to transmit data at the same data rate and/or transmit power for all of the given subchannels. In conjunction, it may be challenging to effectively code and modulate data efficiently for an adaptive loading OFDM system, because of the dynamic transmission parameters. A system that utilizes fixed transmission parameters in some aspects may be simpler to code and modulate, but may be more susceptible to inefficient transmission. Such a system may be Ultra Wide-Band (UWB).
UWB typically transmits each sub-carrier equally loaded with no carrier quality knowledge at the transmitter. Essentially, UWB keeps the average data rate constant. Diversity, and hence interleaving, becomes more important in UWB in order to reduce the chances of losing an information bit. However, equally loading the sub-carriers under-utilizes high quality sub-carriers and may require medium access channel (MAC) mitigation of data losses due to time varying changes in channel conditions.
Therefore, there is a need in the art to provide solutions to the above identified problems.